1. Field of the Invention
The present invention relates to a quartz-based optical fiber with a lens and its manufacturing method, and more particularly, to a quartz-based optical fiber with a lens which feature less aberration of the lens and higher efficiency in coupling to a semiconductor laser and which permits easy positioning of the lens and the semiconductor laser when aligning with the semiconductor laser, and its manufacturing method.
2. Prior Art
A conventional luminous element module to be incorporated in an optical communications system has a lens which is installed between a semiconductor laser, which is a light source, and a quartz-based optical fiber to condense the laser beam into a core of the quartz-based optical fiber. The module is fabricated by aligning the lens, the semiconductor laser, and the core of the quartz-based optical fiber with one another to ensure maximum coupling power between the semiconductor laser and the quartz-based optical fiber, because it is necessary to maximize the efficiency of the coupling between the semiconductor laser and the quartz-based optical fiber.
Recently, a quartz-based optical fiber with a lens, wherein the lens section is formed directly on the end surface of the quartz-based optical fiber, has been proposed. Since this quartz-based optical fiber is equipped with a lens function on its own end surface, no lens is required in fabricating the above-mentioned module. This results in such advantages as a smaller number of parts and less man-hours for the aligning work, contributing to reduced cost.
The quartz-based optical fiber with a lens described above is fabricated, for example, as set forth below. As shown in FIG. 1, a quartz-based optical fiber 1 with its covered portion peeled and exposed is locally heated with a heating means 2 such as a burner, for example, and the heated portion is stretched by pulling it in the directions of arrows "p." The stretching is stopped when a diameter of a stretched section 1a of the quartz-based optical fiber reaches about 10 .mu.m or less, then the stretched section is cut with a cutter, for example.
Then, as shown in FIG. 2, an end section 1b of the quartz-based optical fiber, which has been cut, is heated with a burner 2 to melt it. At this time, the melting end 1b forms a spherical surface due to surface tension. As a result, a quartz-based optical fiber with a lens, the end 1b thereof forms a very small spherical lens, is obtained.
To fabricate an optical module by coupling this quartz-based optical fiber with a lens to a semiconductor laser, as shown in FIG. 3, for instance, a quartz-based optical fiber with a lens is disposed on one side of a fixing base 3 and a semiconductor laser chip 5 such as a LD (laser diode) is disposed on the other side of the fixing base 3, then these two are coaxially brought closer toward each other until the distance between the end section (lens section) 1b of the quartz-based optical fiber with a lens 1 and a luminous surface 5a of the semiconductor laser chip 5, which normally has a diameter of approximately 1 .mu.m, reaches 5 to 10 .mu.m, thus aligning them to ensure a maximum optical coupling power. By using this aligning mode, a coupling efficiency of about 20% to 30% is obtained between the quartz-based optical fiber with a lens 1 and the semiconductor laser chip 5.
In the case of the quartz-based optical fiber with a lens manufactured in the method described above, the diameter of a portion from the lens section 1b at the end to the unstretched portion is tapered. The tapered portion 1c is fairly long, usually a few millimeters, although the length depends on the stretching condition.
Hence, as shown in FIG. 3, when the quartz-based optical fiber with a lens 1 is fixed on one side of the fixing base 3, there will be a fair distance from the fixed section to the luminous surface 5a of the semiconductor laser chip 5. For this reason, if an external force such as vibration is applied to the fixing base 3, the end section (lens section) 1b in the state of correct alignment is displaced with a large vibration width, occasionally causing misalignment between the lens section 1b and the luminous surface 5a. If such misalignment occurs in a fabricated optical module, then the coupling efficiency varies when the optical module is in operation.
Further, in the case of this quartz-based optical fiber with a lens 1, the lens 1b is positioned as a very small sphere on the end of a long tapered section. At the time of fabricating the optical module, to align the lens section 1b with the luminous surface 5a, the leading end of the lens 1b serves as a positioning reference. However, since the end consists of an extremely small curved surface, there is a problem in that it is difficult to clearly identify the position of the end. This occasionally causes the lens section 1b to come into contact with the luminous surface 5a, breaking the luminous surface 5a, when aligning the lens section 1b with the luminous surface 5a.
Furthermore, the method described above requires high skill for setting the conditions for locally heating the quartz-based optical fiber with a lens and the stretching condition. Therefore, the method can hardly be accepted as a method for mass-producing quartz-based optical fibers with lenses with a stable yield.
Under the patent registration No. 1399345, the following quartz-based optical fiber with a lens is described:
The outer peripheral section of a quartz-based optical fiber, which has been machined into a conical shape, is etched with a hydrofluoric acid aqueous solution to cause a conical core to be exposed on one terminal. The top section of the conical core is heated to form the top section into a spherical lens.
Like the one made by melting and stretching, this quartz-based optical fiber with a lens also uses a very small spherical lens formed on the top section as the positioning reference for aligning with the luminous surface of a semiconductor laser. It is, therefore, very difficult to identify the end of the lens, causing the luminous surface to easily break.
Further, "Fibre fresnel Phaseplates with efficient coupling to semiconductor lasers and low reflective feedback" of ECOC, 1990, pp 291 proposes a quartz-based optical fiber with a lens which has a truncated-cone-shaped lens section directly formed on an end surface of a quartz-based optical fiber by immersing the end surface of the optical fiber in a hydrofluoric acid solution.
In this optical fiber, however, the lens section has a larger aberration which results in a lower coupling efficiency, approximately 60%, when it is coupled to a semiconductor laser.
Furthermore, "High-efficiency optical coupling of a single-mode fiber with a heat-treated spherical point" of C-259, Spring Meeting of Electronic Information Communication Society proposes a quartz-based optical fiber with a lens wherein a lens section of the quartz-based optical fiber with a lens proposed in the above-said report is heated by electric discharge to provide the lens section with a smooth surface.
This optical fiber also suffers from a large aberration of the lens section and a low coupling efficiency, approximately 60%, when it is coupled to a semiconductor laser.
Yet further, the laid-open publication of JP patent application No. H3-269403 discloses a method for forming a lens section on the tip of a quartz-based optical fiber by machining the end while rotating the optical fiber around its axis.
However, in a quartz-based optical fiber with a lens manufactured by this method, misalignment between the center of the axial rotation and the center of the core causes decentering between the formed lens section and the core, resulting in a low efficiency of coupling to a semiconductor laser. In addition, the manufacturing yield is low, unavoidably leading to higher cost.